1. Field of the Invention
The invention relates to an active driving circuit system for organic light emitting diode (OLED) and, more particularly, to an OLED active driving system for improving light emission uniformity of an array or flat panel display (FPD) made up of OLEDs by current feedback.
2. Description of the Related Art
Recently, since OLED arrays can generate relatively high luminance of light and have relatively low production and operation costs, they are becoming more and more popular as FPDs. Besides, OLEDs can be fabricated in a variety of sizes from very small (less than a tenth millimeter in diameter) to relatively large (greater than an inch) so that OLED arrays can be fabricated in a variety of sizes. Also, OLED arrays can generate most colors of light with relative ease and provide a very wide viewing angle.
All OLEDs work on the same general principles described as follows. Firstly, one or more layers of organic material are sandwiched between two electrodes. A current is then applied to the OLEDs, causing negatively charged electrons to move into the organic material from the cathode. Positive charges typically referred to as holes move in from the anode. Then, the positive and negative charges meet, combine, and produce photons in the center layers (i.e., the organic material). The color of the photons depends on the electronic properties of the organic material in which the photons are generated.
As disclosed in U.S. Pat. No. 5,748,160, two-dimensional OLED arrays typically contain rows and columns of OLEDs. FIG. 1 shows one of the OLEDs, which is designated by reference numeral 1. Referring to FIG. 1, the OLED 1 is connected to a circuit block 2. The circuit block 2 includes a first transistor 21 having a current carrying electrode 211 connected to a cathode of the OLED 1 and a current carrying electrode 212 connected to ground. The circuit block 2 further includes a second transistor 22 having a current carrying electrode 221 connected to a gate electrode 213 of the first transistor 21. Another current carrying electrode 222 of the second transistor 22 serves as a data signal input terminal 4, and a gate electrode 223 of the second transistor 22 serves as a scan signal input terminal 3. Besides, a capacitor 23 is connected between the gate electrode 213 and ground as a storage element so as to maintain the OLED 1 in an ON mode for a specific period of time, and control the flowing of some fixed current, wherein the current value is determined by the gate-source voltage Vgs of the first transistor 21.
The OLED 1 is addressed by supplying a scan signal to the gate electrode 223 of the second transistor 22, and supplying a data signal to the current carrying electrode 222. Specifically, the scan signal activates the second transistor 22 so that the data signal is input to the gate electrode 213 of the first transistor 21 through the current carrying electrodes 222 and 221. Thereby, the gate electrode 213 is activated. At this time, a current path is completed between the cathode of OLED 1 and ground. Since a supply voltage Vs is connected to the anode of OLED 1, the current flows through the OLED 1, which thus emits light.
OLEDs are typically current driven devices (i.e., emit due to current flowing through them), as opposed to voltage driven devices such as liquid crystal displays (LCDs). Therefore, in an array or FPD made up of OLEDs, it must be assured that each of the OLEDs is driven by the same current under the same supply voltage in order to achieve superior light emission uniformity. However, since the first transistors 21 of the OLEDs do not have the same characteristic parameters, different driving currents can be generated under the same supply voltage. Therefore, the conventional array or FPD made up of OLEDs cannot achieve desirable light emission uniformity.
In view of the above-mentioned requirement for light emission uniformity of OLED array or FPD, the invention provides an OLED active driving system with current feedback. With the OLED active driving system, a driving current for OLED is not affected by variation of characteristic parameters of thin film transistor under an active driving mode, so that the OLED array or FPD can achieve desirable light emission uniformity.
In one embodiment in accordance with the invention, a cathode of an OLED is connected to a current carrying electrode of a first transistor. A current carrying electrode of a second transistor is connected to a gate electrode of the first transistor. Another current carrying electrode of the second transistor serves as a data signal input terminal, and the gate electrode serves as a scan signal input terminal. A capacitor is connected between a gate electrode of the first transistor and ground as a storage element. Two current carrying electrodes of a third transistor are respectively connected to an anode of the OLED and a comparison terminal of a current comparator. A gate electrode of the third transistor is connected to the scan signal input terminal. Two current carrying electrodes of a fourth transistor are respectively connected to the anode of the OLED and a supply voltage. The gate electrode of the fourth transistor serves to receive a reverse signal of the scan signal.
In order to make the driving current input from the third transistor into the OLED not affected by variation of characteristic parameters of a thin film transistor under active driving mode, another comparison terminal of the current comparator is connected to a reference current source for receiving a reference current with predetermined value. The current comparator compares the driving current and the reference current, and then outputs a voltage to the gate electrode of first transistor in response to the comparison result. The gate electrode of the first transistor controls the value of driving current, and therefore the driving current is maintained at the value of reference current due to the feedback effect of the voltage.